http_parser: expose pause/resume method for parser
http: provide backpressure for pipeline flood

If a client sends a lot more pipelined requests than we can handle, then
we need to provide backpressure so that the client knows to back off.
Do this by pausing both the stream and the parser itself when the
responses are not being read by the downstream client.

Backport of 085dd30
crypto: clear errors from verify failure

OpenSSL will push errors onto the stack when a verify fails, which can
disrupt TLS and other routines if we don't clear the error stack

Fixes #6304
crypto: clear openssl error stack when handled

Clear OpenSSL's error stack on return from Connection::HandleSSLError().
This stops stale errors from popping up later in the lifecycle of the
SSL connection where they would cause spurious failures.

This commit causes a 1-2% performance regression on `make bench-tls`.
We'll address that in follow-up commits if possible but let's ensure
correctness first.

Backport of c6e2db2
2013.10.13, Version 0.8.26 (maintenance)

* v8: Upgrade to 3.11.10.26

* crypto: clear openssl error stack when handled (Ben Noordhuis)

* crypto: clear errors from verify failure (Timothy J Fontaine)

* crypto: fix memory leak in LoadPKCS12 (Fedor Indutny)

* http: provide backpressure for pipeline flood (isaacs)

* http_parser: expose pause/resume method for parser (Timothy J Fontaine)

* readline: pause stdin before turning off terminal raw mode (Daniel Chatfield)
Merge branch 'v0.8.26-release' into v0.8
Now working on 0.8.27